1. Field of the Invention
The present invention generally relates to audio apparatus and, more particularly, is directed to a surround audio apparatus.
2. Description of the Prior Art
A surround decoder of Dolby pro-logic (registered trademark) system has been known as a surround audio apparatus.
The surround decoder is hardware that can reproduce such a sound field as the software creator wants to produce with respect to sound of movies, etc. According to the surround decoder, an audio signal transmitted in the form of two channels is decoded to an audio signal of 4 channels while improving a crosstalk characteristic of the audio signal. Further, as an additional function, the surround decoder can create or correct the sound field so that listeners can listen to sound as if they were, for example, in a large concert hall.
An example of a conventional surround decoder will be described with reference to a schematic block diagram forming FIG. 1.
As shown in FIG. 1, there are provided a plurality of analog input terminals 1A to 1N, a plurality of digital input terminals 2A to 2M and input selector switches 13 to 15 which are operated in a ganged relation to each other. A digital-to-analog (D/A) converter 16 is interposed between the input selector switches 15 and 14.
Analog stereo audio signals L, R encoded for surround reproduction or ordinary analog stereo audio signals L, R are respectively supplied to the input terminals 1A to 1N and these stereo audio signals L, R are selected by the selector switches 13, 14.
Similar digital audio signals L, R are respectively supplied to the input terminals 2A to 2M and selected by the selector switch 15. The D/A converter 16 is adapted to convert the digital audio signals applied to the terminals 2A to 2M into analog audio signals because all signal processings such as the surround processing or the like are performed in an analog fashion.
Referring to FIG. 1, the signals L, R from the selector switch 14 are supplied through a balance adjusting circuit 21 and a switch 22 to a Dolby pro-logic decoder 23. The decoder 23 decodes the signals L, R on the basis of the levels and phases of the signals L, R supplied thereto to provide front left and right channel audio signals FL, FR and a front center channel audio signal FC. Also, the decoder 23 derives back left and right channel audio signals BL, BR in cooperation with a decoder 31 which will be described later.
The signals FL, FR are obtained by increasing the levels of the signals L, R in response to the level of the equi-phase component thereof and, the signal FC is mainly the equi-phase component of the signals L, R. Further, the signals BL, BR are mainly obtained from a difference component (L-R) of the signals L, R. The front left and right channel signals FL, FR from the decoder 23 are supplied through a switch 24 to an output terminal 25, and the center channel audio signal FC from the decoder 23 is supplied directly to an output terminal 26. Further, the decoder 23 derives the difference component (L-R) of the signals L, R and this difference component (L-R) is sequentially supplied to the decoder 31 and a delay circuit 32. The decoder 31 has a level expanding characteristic and a high band cutting characteristic, and the delay circuit 32 is adapted to reproduce a reverberation component in the concert hall or the like.
In this way, the delay circuit 32 derives the back left and right channel signals BL, BR and the signals BL, BR are supplied through a switch 33 to an output terminal 34.
As shown in FIG. 1, there is provided a test tone signal generator circuit 29. When the switch 22 is connected to the test tone signal generator 29 side, the test tone signal generator circuit 29 generates a test tone signal which is localized in the left front, right front, center front and rear at every second.
When the switch 24 is connected to the selector switch 14 side, the audio signals L, R selected by the selector switch 14 are directly supplied to the terminal 25.
The Dolby pro-logic system surround decoder is constructed as described above.
In the above surround decoder, however, the balance adjusting circuit 21, the Dolby pro-logic decoder 23, the test tone signal generator circuit 29, the decoder 31 and the delay circuit 32 are constructed by analog discrete parts and analog ICs so that the number of assembly parts is increased, which provides a large-sized and expensive surround audio apparatus.
Further, since the surround decoder is constructed by the analog discrete parts and analog ICs, the surround decoder must be adjusted in the assembly-process. Further, even though such adjustment is performed, aging change occurs frequently.
Furthermore, since the selector switches 13 and 14 are needed to switch the analog audio signals, there is then the disadvantage that the tone quality will be deteriorated by the contacts of the switches 13, 14.